[Enhancement] Improve flow for adding many sources at once (#306)

* Added mechamism for using existing sources as a reference for adding new sources

* Added test

lib/pinchflat_web/controllers/sources/source_html/new.html.heex

@@ -8,7 +8,7 @@
 <div class="rounded-sm border border-stroke bg-white px-5 pb-2.5 pt-6 shadow-default dark:border-strokedark dark:bg-boxdark sm:px-7.5 xl:pb-1">
   <div class="max-w-full">
     <div class="flex flex-col gap-10">
-      <.source_form changeset={@changeset} media_profiles={@media_profiles} action={~p"/sources"} />
+      <.source_form changeset={@changeset} media_profiles={@media_profiles} action={~p"/sources"} method="post" />
     </div>
   </div>
 </div>